1. Field of the Invention
The present invention generally relates to polarizing beamsplitters and associated image display systems, and more specifically to a polarizing beamsplitter that can be used with reflective liquid crystal display systems.
2. Description of the Related Art
U.S. Pat. No. 5,239,322 to Takanashi et al. discloses a conventional image display system in which a readout beam is projected through a collimating lens and onto a polarizing beamsplitter (PBS). The PBS reflects the S-polarized component of the readout beam, which illuminates the face of a spatial light modulator (SLM). The SLM spatially modulates the polarization of the reflected portion of the readout beam in accordance with the spatial intensity pattern of an image. The PBS doubles as a polarization analyzer and spatially modulates the readout beam's intensity in accordance with its polarization modulation. The intensity modulated beam passes through a projection lens, typically implemented as a multi-lens device, which focuses and reimages the intensity modulated beam onto a screen.
The PBS has a single polarizing surface, typically positioned at 45.degree. with respect to the SLM. The thickness of the PBS determines the minimum back focal length (the distance between the SLM and the projection lens) for the system. As the PBS's width, and hence thickness increases to accommodate higher resolution or larger images, the back focal length increases proportionally, thus increasing the display system's overall length. Increasing the focal length causes the light to diverge from the SLM over a larger area, thus requiring larger and heavier lens elements to capture enough light to display the image.
In this configuration, the illumination of the SLM is non-uniform. The collimating lens is not ideal, and thus the portions of the readout beam which pass through its edges are refracted more than the portions which pass through the center of the lens. Therefore the readout beam's intensity is a maximum,at its center and gradually decreases towards its edges. The illumination of the SLM affects the illumination and contrast of the displayed image.
Takanashi discloses a configuration for reducing the back focal length by positioning the projection lens between the SLM and the PBS. The readout beam is passed through a second lens that focuses it onto the PBS. The S-polarized readout beam is reflected towards the projection lens, which collimates the beam and projects it onto the cell. This approach reduces the back focal length but concentrates the beam's optical power onto a small area on the polarizing surface, which can easily damage the PBS. As the luminance, and hence power requirements for image display systems increase, the problem becomes more significant.